Dielectric/metal/dielectric structures using copper as metal and MoO3 as dielectric for use as transparent electrode

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Dielectric/metal/dielectric structures using copper as metal and MoO

₃

as dielectric for use as transparent electrode

I. Pérez Lopéz

^a

, L. Cattin

^a

, D.-T. Nguyen

^a

, M. Morsli

^b

, J.C. Bernède

^c,

⁎

aUniversité de Nantes, Institut des Matériaux Jean Rouxel (IMN), UMR 6502, 2 rue de la Houssinière, BP 92208, Nantes, F-44000 France

bUniversité de Nantes, Faculté des Sciences et des Techniques, 2 rue de la Houssinière, BP 92208, Nantes, F-44000 France

cUniversité de Nantes, Moltech Anjou, UMR 6200, 2 rue de la Houssinière, BP 92208, Nantes, F-44000 France

a b s t r a c t a r t i c l e i n f o

Article history:

Received 30 September 2011 Received in revised form 11 June 2012 Accepted 14 June 2012

Available online 21 June 2012 Keywords:

Transparent conductiveﬁlms Multilayer electrode Molybdenum oxide Copper

Optical properties Electrical properties Organic solar cells

Transparent conductive oxide/metal/oxide, where the oxide is MoO3and the metal is Cu, is realized and characterized. Thefilms are deposited by simple joule effect. It is shown that relatively thick Cufilms are necessary for achieving conductive structures, what implies a weak transmission of the light. Such large thicknesses are necessary because Cu diffuses strongly into the MoO3films. We show that the Cu diffusion can be strongly limited by sandwiching the Cu layer between two Al ultra-thinfilms (1.4 nm). The best structures are glass/MoO3

(20 nm)/Al (1.4 nm)/Cu (18 nm)/Al (1.4 nm)/MoO3(35 nm). They exhibit a transmission of 70% at 590 nm and a resistivity of 5.0 · 10⁻⁴Ωcm. Aﬁrst attempt shows that such structures can be used as anode in organic photovoltaic cells.

1. Introduction

In recent years, organic photovoltaic cell devices (OPVs) have gained a high interest like alternative low-cost source of energy. Moreoverﬂat panel display device (FPD) (like organic light emitting diodes (OLEDs), liquid crystal displays or plasma devices) production has rapidly risen.

For all these optoelectronic devices a transparent electrode is needed.

For instance, it is necessary for hole collection/injection in organic photovoltaic cells and organic lighting emitting devices. Transparent conductive oxides (TCO) are widely used as anode in OPVs or FPD devices.

Among the TCOs, ZnO, SnO2and Indium Tin Oxide (ITO) oxides are already commercialized. These oxides can be made conductive by intrinsic (defects) or extrinsic (foreign atoms) doping. ITO is the most used TCO, since it is, for instance, the most efﬁcient anode in OPVs and OLEDs.

However, its ceramic structure, which limits its application inﬂexible devices[1], and the scarcity of In[2]have resulted in a need of alternative materials research. ZnO and SnO2can also be used as anode in optoelectronic devices[3–7], nevertheless, when they are used as anode in OPV cells or in OLEDs, they introduce an anode/organic material poten- tial barrier. Moreover, in general, to be highly conductive, as needed for electrodes, they need sputter deposition techniques and/or heat post deposition treatments, which are not compatible with organic material[8].

This impedes their use as top or intermediate electrode in organic optoelectronic devices.

Many alternatives have been proposed as transparent electrodes, like semi-transparent metal electrode [9,10], polymer transparent electrode[11,12], and single-walled carbon nanotube[13,14]. How- ever, their low optical transparency when they are conductive, and reciprocally, plus stability difﬁculties in the case of polymer such as Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) still remain a problem.

Therefore, the combination of high transmission in the visible of oxides and the excellent conductivity of metals by sandwiching a thin metal layer between two oxide layers (O/M/O) appears to be an attrac- tive approach. For instance, TCO/Ag/TCO structures have been created to achieve higher conductivity than ITOﬁlms[15–22]. When a metal mirror layer is embedded between two dielectric layers, the dielectric/

metal/dielectric tri-layerﬁlm could suppress the reﬂection from the metal in the visible range and achieve a selective transparent effect.

So, there were more and more studies on the tri-layerﬁlms, such as ITO/Ag/ITO, ZnO/Ag/ZnO and then other structures have been studied [23–27], reaching promising optical and electrical properties. The metal layer should be thin, for high transmittance and continuous for low resistivity. In order to improve the conductivity of transparent con- ductingﬁlms and their optical performances it is necessary to try original materials or constructions. Copper, whose electrical and optical properties are very close to those of silver, and whose price is largely lower, can replace silver in these structures[28–31]. If many works have been

⁎Corresponding author. Tel.: + 33 251125530; fax: + 33 251125528.

E-mail address:jean-christian.bernede@univ-nantes.fr(J.C. Bernède).

doi:10.1016/j.tsf.2012.06.056

Contents lists available atSciVerse ScienceDirect

Thin Solid Films

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / t s f

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devoted to silver as intermediate metal layer, not many works have been published on copper intermediate layer. Moreover, they have been probed only in TCO/Cu/TCO structures. However the low work function of classical TCO results in imperfect work function alignment with hole ex- traction or injection layers in OPVs or OLEDs. Therefore, it could be inter- esting to use oxide which allows better band matching at the interface anode/organic material. MoO3is well known as very efﬁcient hole collecting or injecting layer[32]. Therefore, in the present work, we have studied MoO3/Cu/MoO3(M/C/M) structures. If it has been shown that there is not, or only small, Ag diffusion in the O/Ag/O structure[33,34], Cu diffusion has been suggested[28]but never checked. Therefore in the present work we have focused on the study of the Cu diffusion in MoO3embed- ding layers and its effect on optical and electrical properties of MoO₃/ Cu/MoO3(M/C/M) structures. After noting the strong diffusion of copper into MoO3layers, we show the effectiveness of aluminum as a diffusion barrier.

2. Experimental details

MoO3/Cu/MoO3(MCM) structures were deposited, on soda lime glass substrates using a simple joule effect evaporation system. The multilayer films were successively deposited onto the glass substrates without vacuum break, using tungsten crucibles. The tungsten crucibles were loaded with MoO3powder or Cu wire. The Al was deposited using a tungsten wire. As a matter of fact, the experimental study below shows that Al is an efficient buffer layer to prevent Cu diffusion and therefore the optimum structures are MoO3/Al/Cu/Al/MoO3(MACAM). The substrate temperature during deposition was room temperature. Deposition rates and film thicknesses were measured in situ by quartz monitor. The deposition rate of the MoO3was 0.10 nm.s⁻¹, 0.20 nm.s⁻¹for the Cu and 0.05 nm.s⁻¹for the Al. The thicknesses of the MoO3films werefixed at 20 nm and 35 nm for the bottom and the top layer respectively.

The structures have been characterized using different techniques.

X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the surface of the structures as well as to perform the composition proﬁles using sputtering with argon gas. XPS analyses were performed with a magnesium X-ray source (1253.6 eV) operating at 12 kV and 10 mA (Leybold LHS12). During the measurements, the vacuum was 10⁻⁷Pa and the pass energy for high resolution spectra was 50 eV. The samples were grounded with conductive paste to decrease the charge effect. The quantitative studies were based on the de- termination of each peak after subtraction of s-shaped background. For the quantitative study, the sensitivity factors, s, given by the manufac- turer were: Mo3d s = 2.5, Al2s s = 0.15, Al2p s = 0.12, Cu2p s = 6.3 and O1s s = 0.6. The depth proﬁle of the structures was studied by re- cording successive XPS spectra obtained after argon ion etching for short periods. Sputtering was accomplished at pressures of less than 5 × 10⁻⁴Pa, a 10 mA emission current and a 3 kV beam energy using an ion gun. With these experimental conditions, all the surface of the sample was sputtered.

The optical measurements were carried out at room temperature using a Carry spectrometer. The transmission was measured at wave- lengths of 1.3 to 0.29μm.

The majority carrier type has been checked by the hot probe tech- nique. An n-type constantan wire was used as the reference sample.

The electrical resistivity and Hall mobility of theﬁlms were deter- mined by Hall effect measurements in a van der Pauw conﬁguration.

The morphology of the different structures used as anode was ob- served through scanning electron microscopy (SEM) with a JEOL 7600F at the“centre de microcaractérisation de l'Université de Nantes”. The im- ages were realized in secondary and backscattering mode (Operating voltage 10 kV).

Details of the preparation of organic solar cells are given in[35]. The electron donor used was copper phthalocyanine (CuPc), the electron acceptor was fullerene (C60), and the exciton blocking layer was bat- hocuproine (BCP). CuPc, C60and BCP have been deposited in a vacuum

of 10⁻⁴Pa. The thinfilm deposition rates and thickness were estimated in situ with a quartz monitor. The deposition rate andfinal thickness were 0.05 nm/s and 35 nm in the case of CuPc, 0.05 nm/s and 40 nm in the case of C60and 0.1 and 9 nm in the case of BCP. After organic thin film deposition, the aluminum top electrodes were thermally evaporated, without breaking the vacuum, through a mask with 2 mm ×8 mm active areas. This Alfilm behaves as the cathode, while the optimum MACAM structure is the anode. Finally the structures were:

glass=anode=CuPc 35 nmð Þ=C₆₀ð40 nmÞ=BCP 9 nmð Þ=Al 120 nmð Þ:

with anode= MoO3/Al/Cu/Al/MoO3 or MoO3/Al/Cu/Al/MoO3/Au. An ultra thin gold buffer layer was introduced in some cells, because we have shown previously that it is very efﬁcient as hole collecting buffer layer[3,4,32].

The characteristics of the photovoltaic cells were measured using a calibrated solar simulator (Oriel 300 W) at 100 mW/cm²light in- tensity adjusted with a reference cell (0.5 cm²CuInGaSe2solar cell, calibrated at NREL, USA). Measurements were performed at an ambi- ent atmosphere. All devices were illuminated through TCO electrodes.

We would like to stress here that we do not intend to achieve the best overall cell performance. We rather show that MoO3/Al/Cu/Al/MoO3

structures can be used as anode in organic solar cells.

3. Experimental results

In thefirst attempt, the Cu thickness has been used as parameter. The thicknesses of the MoO3films were 20 nm for the bottom and 35 nm for the top layer. The thinnest copper thickness that allowed a MCM conductive structure was 32 nm. Due to this large thickness, the structure ex- hibits a low transmittance (Fig. 1). XPS quantitative surface analyze of a structure, glass/MoO₃/Cu/MoO₃(20 nm/24 nm/35 nm), which is not conductive shows the presence of more than 10 at.% of copper around the surface of the structure. The depth profile of this structure is shown inFig. 2. It can be seen that Cu diffused through all the thickness of the structure. There are between 4 and 10 at.% of Cu in the top layer and around 20 ±5 at.% in the bottom MoO3layer. The high Cu diffusion justifies the need of a Cufilm thick of, at least, 32 nm to obtain conductive structures. It is clear that, in order to improve the MCM anode structure, it is necessary to decrease the Cu diffusion. Therefore we introduce a copper diffusion barrier in the MCM structure. Since the transmittance of the structure is poor, it is preferable to introduce a transparentfilm as diffusion barrier. In this purpose, we use an Alfilm thick of 1.4 nm. In the vacuum conditions used during this work, it is clear that such thin Alfilm forms an aluminum oxidefilm which is transparent (optically

400 600 800 1000 1200

0 10 20 30 40 50 60 70 80 90 100

Transmittance (%)

(nm)

c glass/M (20 nm)/A (1.4 nm)/C (35 nm)

b glass/M (20 nm)/A (1.4 nm)/C (32 nm)/M (35 nm) a glass/M (20 nm)/C (32 nm)/M (35 nm)

a b c

Fig. 1.Transmittance spectra of the different conductive structures: a Glass/MoO3

(20 nm)/Cu (32 nm)/MoO3(35 nm). b Glass/MoO3(20 nm)/Al (1.4 nm)/Cu (32 nm)/

MoO3(35 nm). c Glass/MoO3(20 nm)/Al (1.4 nm)/Cu (32 nm).

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and electrically)[36,37]. Firstly, in order to determine what MoO3layer, that of the top or of the bottom, which induces the largest conductivity decrease because of the copper diffusion, we realized glass/MoO3/Cu and glass/Cu/MoO3bi-layer samples, with Cu and MoO3thicknesses of 32 and 35 nm respectively. As expected, since, as shown by the XPS proﬁle (Fig. 2a) the Cu diffusion is higher in the bottom layer, the Glass/

Cu/MoO3sample was conductor whereas the glass/MoO3/Cu was not.

Hence we added the Al buffer layer between the MoO3bottom layer and the copper layer. The glass/MoO3/Al/Cu structures remain conductive until a minimal Cu thickness of 18 nm. It can be seen by the naked eyes that the deposition of the thin Alfilm (1.4 nm) between the MoO3and the Cufilm modifies significantly the Cu diffusion. Indeed, in the presence of Al, the sample appears pink, while without Al it is gray (InsetFig. 3).

More precisely, by SEM visualization, one can see inFig. 3a that, without Al diffusion barrier, the copperfilm is formed by round shaped nanorods with diameter of about 15 nm all over the MoO3film. There is no percolation between these nanorods of Cu, which justifies that the structures without Al diffusion barrier are not conductive. When the Al diffusion barrier is introduced in the structure (Fig. 3b), the Cufilm appears more uniform, with continuing paths all along thefilm, which justifies the conductivity of thefilms. Therefore XPS and SEM studies are in good agree- ment, without Al buffer layer there is a strong Cu diffusion in the MoO3

ﬁlms, the Cuﬁlm is not continuous and the structures are resistive.

So we have grown glass/MoO3/Al/Cu/MoO3structures. The transmission of these structures, with and without MoO3top layer is shown in Fig. 1, using a copper layer thickness of 32 nm. First of all, the structures being lighted from the side of the top layer, it can be seen that the presence of the MoO3top layer strongly increases the transmittance of the structure. Moreover, the thickness of the copperﬁlm being the same for each sample, the presence of the Al diffusion barrier allows a signif- icant increase of the complete structure transmission. With such glass/

MoO3/Al/Cu/MoO3structures, the Cuﬁlm thickness necessary to achieve conductive sample decreases to 24 nm. The transmittance of such glass/

MoO3(20 nm)/Al (1.4 nm)/Cu (24 nm)/MoO3(35 nm) structure is reported inFig. 4. It is far higher than that of the conductive structure without diffusion barrier (Fig. 1). It should be noted that, while in the case of glass/MoO3/Al/Cu the samples remain conductive until a minimal Cu thickness of 18 nm, 24 nm is necessary for glass/MoO3/Al/Cu/MoO3. The Al diffusion barrier allows reducing of a quarter the thickness of the copper layer which is necessary to obtain conductive structures.

However the XPS study shows that the atomic Cu concentration at the surface of the structures still rises to nearly 20 at.%. Indeed, the superpo- sition of the Cu depth proﬁles (Fig. 5) shows that, in the case of glass/

MoO3/Al/Cu/MoO3there is a translation of the Cu proﬁle from the bottom MoO3layer to the top MoO3layer. Therefore, when an Al diffusion barrier is introduced between the MoO3bottom layer and the copper, the aluminum blocks, at least partly, the Cu diffusion toward this bottom

layer. However, the full width at half maximum (FWHM) of the Cu peak is nearly the same than that without Al diffusion barrier, which means that Cu diffusion has been shifted towards the top MoO3ﬁlm. Therefore we have added a second diffusion barrier, between the MoO3top layer and the Cu layer. The addition of the second diffusion barrier led to structures glass/MoO3/Al/Cu/Al/MoO3(MACAM). Such structures are conductive with a copperﬁlm thickness equal to the half of that which is necessary for structures without Al buffer layer, that is to say: 20 nm/

1.4 nm/16 nm/1.4 nm/35 nm for MACAM structure and 20 nm/32 nm/

35 nm for MCM. XPS was performed on the structure MACAM (20 nm/

1.4 nm/21 nm/1.4 nm/35 nm). The XPS depth proﬁles ofFig. 5shows that the MACAM structures allows to achieve a Cu peak with the smallest FWHM, which means that Cu diffusion towards both MoO3layers is limited by the presence of the Al buffer layers. We can also note, as shown by Cu2p XPS spectra, that in the bulk of the structure, that is to say, after 30 s of etching, the copper is present in the MoO3layer in the metallic form, which means that the diffused copper is not oxidized.

The DRX diagram shows a broad and weak reﬂection peak around 2θ= 25°, indicating that the structures are amorphous.

Concerning the optical properties, the glass/MoO₃/Al/Cu/Al/MoO₃ structures (20 nm/1.4 nm/21 nm/1.4 nm/35 nm) exhibit a transmission of 70% at 590 nm (Figs. 4, 6), with a broad region (375 nm to 750 nm) where it is higher than 60%. The transmission of the MoO₃/ Al/Cu/Al/MoO3structures increases with the increase of the Cu thickness from 14 to 18 nm, for thicker Cu layers, it decreases. With the increase of the Cu layer thickness there is also a small red shift of the transmittance peak. It has been reported, in the case of dielectric/

silver/dielectric, that the high index of refraction difference between Ag and the dielectric layer results in efﬁcient plasmon cou- pling such that visible transparency greater than 80% can be achieved[15,38]. In the same way, the optimum transmittance of 0 20 40 60 80 100 120 140 160 180 200

0 10 20 30 40 50 60 70 80 90 100

Atomic concentration (%)

Etching time (min) Mo Cu O

Fig. 2.XPS depth proﬁles of a glass/MoO3(20 nm)/Cu (24 nm)/MoO3(35 nm) structure.

Fig. 3.SEM visualization of the surface of (a) glass/MoO3(20 nm)/Cu (18 nm) structures and (b) glass/MoO3(20 nm)/Al (1.4 nm)/Cu (18 nm). InsetFig. 3: photographs of (c) glass/MoO3(20 nm)/Cu (18 nm) structures and (d) glass/MoO3(20 nm)/Al.4 (1 nm)/Cu (18 nm).

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MACAM structures, when the copper thickness is 18 nm, can be attributed to surface plasmon resonance of properly designed Cu layer.

Further Cu layer thickness increase results in a decrease in transmittance, the copperﬁlm becoming homogeneous, which indicates that the effective surface plasmon resonance of the Cu layer occurs at the transition region from aggregated Cu layer to a continuous layer.

It should be underlined that the optimum transmission domain of the MACAM structures matches well the solar spectrum (Fig. 4). More- over the absorption domain of CuPc, the photoconductiveﬁlms of the organic cells, is mainly situated in the region of large transparency of MACAM structures.

The structures are n-type. The electrical conductivity of thefilms varies strongly with the Cu layer thickness. Films are systematically metallic-like when the Cu layer is 16 nm thick or more, while they are nearly insulating for smaller thickness (Table 1). Therefore the passage from discontinuous to continuousfilms corresponds to the threshold percolation of path formation, which explains the sudden resistivity value commutation from 6.2 × 10⁴to 1.6× 10⁻³Ωcm. Above the percolation thickness value, the conductivity increases slightly with the copper layer thickness. However, Cufilm thickness larger than 18 nm results in a decrease in transmittance. The trade off between the sheet resistance and the transmittance of the MACAM structure shows that the optimum structure is obtained with a silver thickness of 18 nm.

We have estimated theﬁgure of meritΦTCof the MACAM structures in order to compare them to that of other indium free electrode, as de- ﬁned by Haacke[39]:

ΦTC¼T¹⁰=Rsh;

where T is the transmittance, and Rsh is the sheet resistance Rsh = 1/σt, and t thickness of the structure. The value achieved is ΦTC= 0.43 · 10⁻³Ω⁻¹.

In thefield of transparent electrodes, which are In free, this result is better or of the same order of magnitude than those achieved with semitransparent metal films. Actually, in the case of CuNi bilayer structures[9]and Au with optimized morphology[10], thefigure of merit, deduced from the optical and electrical measurements, is ΦTC≈0.23 · 10⁻³Ω⁻¹, while it isΦTC= 0.6 · 3 · 10⁻³Ω⁻¹in the case offlexible Ag electrode[40]. However when Ag is used in the structures MoO3/M/MoO3, the value obtained isΦTC= 1.3 · 10⁻³Ω⁻¹[23]. The difference is due to the higher value of the maximum transmission (80%), which is due to the thickness of Cu needed to obtain the percolation of the metalfilm. This is related to the resilience of the Cu diffusion.

400 600 800 1000 1200

0 20 40 60 80

Transmittance (%)

14 nm 16 nm 18 nm 20 nm Cu thickness

(nm)

Fig. 6.Transmittance spectra of the Glass/MoO3(20 nm)/Al (1.4 nm)/Cu (20 nm)/Al (1.4 nm)/MoO3(35 nm) structures, with different Cu layer thickness.

400 600 800 1000 1200

0 20 40 60 80 100

Transmittance (%)

Glass/M (20 nm)/C (32 nm)/M (35 nm).

Glass/M (20 nm)/A (1.4 nm)/C (24 nm)/ M (35 nm).

Glass/M(20 nm)/A(1.4 nm)/C(20 nm)/A(1.4 nm)/M (35 nm),

a

b

(nm)

400 600 800 1000 1200 1400

arb. units

AM 1,5 solar spectrum optical density of CuPc

Fig. 4.Transmittance spectra of the different structures (a), AM1.5 solar spectrum and optical density spectrum of CuPc (b).

0 50 100 150 200

0 10 20 30 40 50 60 70 80

Atomic concentration (%)

Etching time (min) MCM MACM MACAM b

a

c

Fig. 5.Cu depth proﬁles of the different structures: a Glass/MoO3(20 nm)/Cu (24 nm)/

MoO3(35 nm). b Glass/MoO3(20 nm)/Al (1.4 nm)/Cu (24 nm)/MoO3(35 nm). c Glass/

MoO3(20 nm)/Al (1.4 nm)/Cu (24 nm)/Al (1.4 nm)/MoO3(35 nm).

Table 1

Electrical characteristics of MACAM structures.

Cu thickness (nm) 14 16 18 20

Resistivity (Ωcm) 6.2 · 10^{+ 4} 1.6 · 10⁻³ 5.0 · 10⁻⁴ 1.4 · 10⁻⁴

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Therefore in the future, it will be necessary to minimize more the Cu diffusion.

Nevertheless, the optima MACAM structures have been probed as anode in OPV cells. Different anode configurations, issued from our previous studies have been probed[32]. The best results are obtained with MACAM anodes covered with an ultra thin Au layer (Fig. 7). Up to now, when a bare MACAM structure is used, the cells exhibit very small open circuit voltage (Voc≈0.2 V). The characterization of the MACAM multilayer structures has shown that copper is present at the surface, which can explain the small Voc values of cells using these anodes. In- deed, we have already shown that, if we introduce an ultra thin gold film between the ITO anode and the organicfilm, high efficiency solar cells are obtained, while it is not the case when we introduce an ultra thin copperfilms, because the open circuit voltage of the cells is small [41].

4. Conclusion

We have studied the properties of dielectric/metal/dielectric structures, with MoO3as dielectric and Cu as metal. Contrarily to the results achieved with sputtered ZnO[28], in the case of evaporated MoO3, there is a large Cu diffusion into the oxide layers. This increases significantly the Cu layer thickness necessary to achieve conductive structures and therefore decreases thefilm transmittance. We show that, after intro- ducing Al diffusion barriers, the thickness of the copper layer necessary to achieve conductivefilms, can be reduced to half of the thickness rel- ative to the thickness without the aluminum buffer layers. It is shown that the threshold thickness value is 16 nm, where the structures com- mute from an insulating state to a conductive state. We attribute this commutation to the percolation of conducting copper paths. It is shown that the transmittance of thefilms increases when the copper thickness increases from 14 to 18 nm, while further increase induces transmittance decrease. Such effect is attributed to surface plasmon resonance. There- fore the structures with the best performances are obtained when the copper thickness is 18 nm. These MACAM structures (20 nm/1.4 nm/

21 nm/1.4 nm/35 nm) exhibit the same conductivity than commercial ITO, while they present a maximum transmission of 70%. In the future, for better transmittance, the Cu diffusion should be minimized. They have been probed as anode in organic solar cells. We demonstrated that MACAM structures can be used as anode in indium free organic solar cells. Work is underway in the laboratory to improve the surface proper-

ties of these structures in order to beneﬁt of the properties of CuO. Indeed recent work has shown that while copper has a negative effect on cell performance, the CuO can improve them signiﬁcantly[42].

Acknowledgments

OTC-2011 project was supported by the network Nanorgasol Mission Resources and Technological Skills CNRS.

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-200 0 200 400 600

-2 -1 0 1 2 3

J(mA/cm2)

V (mV)

Under light AM 1.5 In the dark

η= 0.22%

Jsc = 2.04 mA/cm² Voc = 0.38 V FF = 29%

Fig. 7.Typical J–V characteristics of glass/MACAM/Au//CuPc/C60/BCP/Al structures, in the dark (full symbol) and under illumination of AM1.5 solar simulation (100 mW/cm²) (open symbol).